Advances in magnetic resonance (MR) perfusion imaging have provided clinical researchers with the opportunity to measure quantitative regional increases in cerebral blood flow. The purpose of this study is to acquire the technical experience required to perform MR perfusion imaging studies of the hypercapnic cerebral blood flow response. Cerebral blood flow will be increased by inhalation of carbogen (an air mixture containing 6 percent CO2 and used to callabrate experiments for determining oxygen consumption. The technical experience obtained in this study will be used to design a study of the pharmacological and physiological mechanisms underlying cerebral blood flow increases during hypercapnia.